<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  .box{
    width: 500px;
    height:200px;
    background-color: yellow;
  }
  .zfx{
    width: 50%;
    padding-top: 50%;
    background-color: skyblue;

  }
</style>
<body>
  <div class="box">
    <div class="zfx"></div>
  </div>
</body>
</html>